I manage to trace issue to code
//Filter results.
if ( $filter && $posts ) {
var_dump($filter);
return $this->filter_posts( $posts, $filter );
In the file class-acf-internal-post-type.php, within the get_posts function, I believe the post_id used in the filter is invalid, as it points to a media post ID rather than an ACF post. I don’t know the internals of ACF, but after removing it, the ACF values appeared in the JSON API. However, this may break other things.
